A major problem which limits the usefulness and useful lifetime of materials exposed to ultraviolet (UV) radiation is degradation associated with such exposure. This degradation results from decomposition and other chemical reactions of organic compounds in the materials initiated by absorption by such compounds of photons of ultraviolet light. Materials exposed to sunlight are especially susceptible to degradation due to UV-exposure, because a significant component of sunlight is UV-radiation. Among such materials are paints, plastics, fabrics and dyes.
In U.S. Pat. No. 4,902,108, which is incorporated herein by reference, solutions of electrochromic compounds are described. These solutions are useful as the media of variable transmittance in single-compartment, self-erasing, solution-phase electrochromic devices. The devices, in turn, are useful as the variable transmittance components in variable transmission light filters, such as windows, and variable reflectance mirrors, such as anti-glare rearview mirrors in automobiles. Components of the solutions of electrochromic compounds described in the aforementioned patent, especially the electrochromic compounds themselves, readily absorb UV photons and, as a consequence, undergo degradative reactions upon exposure to UV light. Thus, windows and rearview mirrors, wherein such solutions are employed to provide variable transmittance or reflectance, have useful lifetimes significantly limited in environments, such as on the outside walls of buildings or on the outside of automobiles, where they are exposed for extended periods of time to ultraviolet light, as from the sun.
There has existed a need, then, to stabilize materials, which comprise UV-radiation-absorbing organic compounds, against degradation caused by exposure to UV-light. Many ultraviolet stabilizer agents, which are compounds which provide such stabilization, when combined with compounds susceptible to UV-degradation, are known in the art. Ultraviolet stabilizer agents absorb ultraviolet radiation competitively with other compounds in a stabilized composition or are capable, without decomposition of the stabilizer itself, of dissipating the energy acquired upon absorption of a UV-photon by the materials being stabilized. Such agents must also satisfy a number of other requirements, depending on properties of the compositions to be stabilized. For example, the agents must be sufficiently soluble in such a composition and must not interact with other components in the composition in a way that interferes with using the composition for its intended purpose. Thus, an ultraviolet stabilizer agent for use in a plastic must not, at a concentration high enough to provide the desired stability, adversely affect, for example, the strength of the plastic. Similarly, an ultraviolet stabilizer agent for use in a solution of electrochromic compounds employed as a medium of variable transmittance to visible light in an electrochromic device must not interact with the electrochromic compounds in a way that so significantly reduces their changes in absorbance to visible light upon oxidation or reduction that they cease to function effectively as electrochromic compounds and must not compete with the electrochromic compounds for oxidation or reduction at the electrodes of an electrochromic device.
Among ultraviolet stabilizer agents, known to stabilize plastics, are the compound ethyl-2-cyano-3,3-diphenyl acrylate, sold by BASF (Parsippany, N.J., USA) under the trademark Uvinul N-35 and by Aceto Corporation (Flushing, N.Y., USA) under the trademark Viosorb 910; the compound (2-ethylhexyl)-2-cyano-3,3-diphenyl acrylate, sold by BASF under the trademark Uvinul N-539; the compound 2-(2-hydroxy-4-methyl-phenyl)benzo[d]triazole, sold by Ciba-Geigy Corp. under the trademark Tinuvin P; the compound 2-hydroxy-4-methoxybenzophenone, sold by American Cyanamid under the trademark Cyasorb UV 9; and the compound 2-ethyl-2'-ethoxyoxalanilide, sold by Sandoz Color & Chemicals under the trademark Sanduvor VSU.
The present invention is directed to stabilization against degradation upon exposure to UV-radiation of solutions, such as those described in U.S. Pat. No. 4,902,108, which are useful as the media of variable transmittance in electrochromic devices, especially single-compartment, self-erasing, solution-phase electrochromic devices.